The present application is related to and claims priority on Japanese Patent Applications 11-370,602, filed on Dec. 27, 1999, 2000-58646, filed on Mar. 3, 2000, and 2000-102473, filed on Apr. 4, 2000, the entire contents of each of which are hereby incorporated herein by reference.
1. Field of the Invention
The present invention relates to an arrayed waveguide grating type optical multiplexer/demultiplexer and an optical waveguide circuit which are used in the field of optical communications and similar fields.
2. Discussion of the Background
Recently, in optical communications research and development of optical wavelength division multiplexing communications has actively been pursued as a way to exponentially increase transmission volume, and the results are being put into practice. Optical wavelength division multiplexing communications uses, for example, a technique of wavelength division multiplexing on a plurality of light beams each having a different wavelength from one another to transmit them. For systems using such optical wavelength division multiplexing communications, an optical multiplexer/demultiplexer is necessary which demultiplexes light that has undergone wavelength division multiplexing to be transmitted to create a plurality of light beams each having a different wavelength from one another, and which multiplexes a plurality of light beams each having a different wavelength from one another.
An optical multiplexer/demultiplexer of this kind preferably has the following capabilities. Firstly, it should be capable of multiplexing and demultiplexing light with a wavelength interval as narrow as possible within a range of a preset wavelength. Secondly, it should be excellent in wavelength flatness in the vicinity of the central wavelength of light to be multiplexed/demultiplexed. Thirdly and lastly, it should have low crosstalk between one passing wavelength and another passing wavelength adjacent thereto (hereinafter referred to as adjacent crosstalk).
Of the desired capabilities listed above, the first capability is met by, for example, an arrayed waveguide grating (AWG) type optical multiplexer/demultiplexer. An arrayed waveguide grating type optical multiplexer/demultiplexer such as shown in FIG. 19A, for example, is obtained by forming on a substrate 11 an optical waveguide unit 10 that has a waveguide structure.
The above waveguide structure includes one or more optical input waveguides 12 arranged side by side, a first slab waveguide 13 connected to the output ends of the optical input waveguides 12, an arrayed waveguide 14 connected to the output end of the first slab waveguide 13, a second slab waveguide 15 connected to the output end of the arrayed waveguide 14, and a plurality of optical output waveguides 16 arranged side by side and connected to the output end of the second slab waveguide 15.
The arrayed waveguide 14 propagates light that is output from the first slab waveguide 13, and is a plurality of channel waveguides 14a arranged side by side. Lengths of adjacent channel waveguides 14a are different from each other with the differences (xcex94L) preset. The optical input waveguides 12 and the optical output waveguides 16 have the same dimensions.
The number of optical output waveguides 16 is determined, for example, in accordance with how many light beams having different wavelengths from one another are to be created as a result of demultiplexing of signal light by the arrayed waveguide grating type optical multiplexer/demultiplexer. The channel waveguides 14a constituting the arrayed waveguide 14 are usually provided in a large number, for example 100. However, FIG. 19A is simplified and the numbers of the channel waveguides 14a, the optical output waveguides 16, and the optical input waveguides 12 in FIG. 19A do not reflect the actual numbers thereof.
FIG. 19B schematically shows an enlarged view of an area of FIG. 19A surrounded by the chain line A. As shown in FIG. 19B, in the arrayed waveguide grating type optical multiplexer/demultiplexer of the background art, straight output ends of the optical input waveguides 12 are connected directly to the input end of the first slab waveguide 13. Similarly, the straight entrance ends of the optical output waveguides 16 are connected directly to the output end of the second slab waveguide 15.
The optical input waveguides 12 are connected to, for example, transmission side optical fibers (not shown), so that light having undergone the wavelength division multiplexing is introduced to the optical input waveguides 12. The light, after traveling through the optical input waveguides 12 and being introduced to the first slab waveguide 13, is diffracted by the diffraction effect thereof and is input to the arrayed waveguide 14 to travel along the arrayed waveguide 14.
After traveling through the arrayed waveguide 14, the light reaches the second slab waveguide 15 and then is condensed at the output end of the second slab waveguide 15. Because of the preset differences in lengths between adjacent channel waveguides 14a of the arrayed waveguide 14, light beams after traveling through the arrayed waveguide 14 have different phases from one another. The phase fronts of many light beams from the arrayed waveguide 14 are tilted in accordance with this difference and the each position where the each light beam is condensed is determined by the angle of this tilt. Therefore, the light beams having different wavelengths are condensed at different positions from one another. By forming the optical output waveguides 16 at these positions, the light beams having different wavelengths can be output from their respective optical output waveguides 16 provided for the different respective wavelengths.
For instance, as shown in FIG. 19A, light beam having undergone the wavelength division multiplexing and having wavelengths of xcex1, xcex2, xcex3, . . . , xcexn (n is an integer equal to or larger than 2) is input to one of the optical input waveguides 12. The light beam is diffracted in the first slab waveguide 13, reach the arrayed waveguide 14, and travel through the arrayed waveguide 14 and the second slab waveguide 15. Then, as described above, the light beams are respectively condensed at different positions determined by their wavelengths, enter different optical output waveguides 16, travel along their respective optical output waveguides 16, and are output from the output ends of the respective optical output waveguides 16. The light beams having different wavelengths are output through optical fibers (not shown) connected to the output ends of the optical output waveguides 16.
In this arrayed waveguide grating type optical multiplexer/demultiplexer, an improvement in wavelength resolution of a grating is in proportion to the differences in lengths (AL) between the adjacent channel waveguides 14a of the arrayed waveguide 14. When the optical multiplexer/demultiplexer is designed to have a large xcex94L, it is possible to multiplex/demultiplex light to accomplish wavelength division multiplexing with a narrow wavelength interval. However, in the background art there are limits to how narrow a wavelength interval can be multiplexed. The optical multiplexer/demultiplexer has a function of multiplexing/demultiplexing a plurality of signal light beams. A function of demultiplexing or multiplexing a plurality of optical signals with a wavelength interval of 1 nm or less is deemed necessary for optical wavelength division multiplexing communications of high density.
In order for the above arrayed waveguide grating type optical multiplexer/demultiplexer to practice the second desired capability of the optical multiplexer/demultiplexer, i.e., to achieve central wavelength flatness, and to broaden the 3 dB band width (3 dB pass band width) of optical central wavelengths output from the optical output waveguides 16, an arrayed waveguide grating type optical multiplexer/demultiplexer having a structure as shown in FIGS. 20A and 20B has been proposed. This arrayed waveguide grating type optical multiplexer/demultiplexer proposed has, as shown in FIG. 20A, substantially the same structure as the background arrayed waveguide grating type optical multiplexer/demultiplexer illustrated in FIG. 19A except that, and as shown in FIGS. 20B and 21, the output ends of the optical input waveguides 12 have a different structure.
The arrayed waveguide grating type optical multiplexer/demultiplexer of FIGS. 20A and 20B is disclosed in Japanese Patent Application Laid-open No. Hei 8-122557. In this arrayed waveguide grating type optical multiplexer/demultiplexer, a slit-like waveguide 50 is formed at the output end of each of the optical input waveguides 12. The slit-like waveguide 50 has, as shown in FIG. 21, a tapered waveguide portion 2A whose width gradually increases with a taper angle xcex8. The tapered waveguide portion 2A has at its center a trapezoidal slit 19, so that two narrow waveguide portions 81, 82 which are spaced apart from each other are formed. The distance between the narrow waveguide portions 81, 82 is gradually increased toward the first slab waveguide 13 in FIG. 21, with the left-hand distance labeled as CW and the right-hand distance labeled as SW.
According to this proposed arrayed waveguide grating type optical multiplexer/demultiplexer as disclosed in Japanese Patent Application Laid-open (Kokai) No. Hei 8-122557, the 3 dB band width of light to be multiplexed and demultiplexed by the arrayed waveguide grating type optical multiplexer/demultiplexer can be broadened. This can be confirmed by, for example, loss wavelength characteristics shown in FIG. 22.
Other waveguide structures for arrayed waveguide grating type optical multiplexers/demultiplexers have also been proposed. One such other waveguide structure, as shown in FIG. 23, has a parabolic tapered waveguide 20 connected to the output end of each of the optical input waveguides 12. Another waveguide structure as shown in FIG. 24, has a multi-mode interface waveguide 21 connected to the output end of each of the optical input waveguides 12.
The structure shown in FIG. 23 is the structure of an arrayed waveguide grating proposed by NTT in Japanese Patent Application Laid-open (Kokai) No. Hei 9-297228. The structure shown in FIG. 24 is a structure proposed by Bell Communication Research Inc. in U.S. Pat. No. 5,629,992 titled xe2x80x9cPassband Flattening of Integrated Optical Filtersxe2x80x9d.
However, the structure of the arrayed waveguide grating type optical multiplexer/demultiplexer having the slit-like waveguide 50 as shown in FIGS. 20A, 20B, and 21 is not simple and, hence, a problem arises in that variations in production are likely to result, and accordingly variations in abilities result. In addition, although this arrayed waveguide grating type optical multiplexer/demultiplexer does broaden the 3 dB band width, when the 3 dB band width is broadened in order to widen the 1 dB band width which is another measure of the wavelength flatness, ripple (see, for example, area B in FIG. 22), which is yet another measure of the wavelength flatness, is increased. It is also found that the 1 dB band width is divided into two at this ripple resulting in a narrowed 1 dB band width, and that the adjacent crosstalk, which is the third desired capability of the optical multiplexer/demultiplexer, is degraded.
The present invention has been made to address the above and other problems in the background art, and an object of the present invention is therefore to provide an arrayed waveguide grating type optical multiplexer/demultiplexer in which the 1 dB band width is wide, the ripple is small, the adjacent crosstalk is low, and the production yield is high.
To achieve the above and other objects, the present invention is directed to an optical waveguide which may be utilized in an array waveguide grating optical multiplexer/demultiplexer. According to the present invention, the optical waveguide is a single mode waveguide and a multi-mode waveguide configured to realize multi-mode and connected to the single mode waveguide. The multi-mode waveguide is a multi-mode broadening waveguide which has a width which increases toward the direction toward the arrayed waveguide. The multi-mode broadening waveguide may have a trapezoidal shape and may be connected to a constant width waveguide which has the same width as that of the upper base of a trapezoidal waveguide. Further, the multi-mode waveguide itself may include a constant width waveguide which is connected to the single mode waveguide.
Thus, to achieve the above and other objects, the present invention places a multi-mode waveguide whose width increases toward an arrayed waveguide, such as a trapezoidal waveguide on an output end of an optical input waveguide, for example. The present invention thus can provide an arrayed waveguide grating type optical multiplexer/demultiplexer in which the 1 dB band width is wide, ripple is small, and adjacent crosstalk is low.
A straight waveguide narrower than the optical input waveguide may be placed between, for example, the optical input waveguide and the trapezoidal waveguide. Because of this straight waveguide, if the optical input waveguide has a curved portion and the central position of the light intensity distribution is deviated from the center in width of the optical input waveguide after the light has traveled through this curved portion, the central position of the light intensity distribution can be moved to the center of the straight waveguide as the light travels along the straight waveguide. The light intensity center thus can be input in the center in width of the trapezoidal waveguide.
The arrayed waveguide grating type optical multiplexer/demultiplexer is also formed by utilizing the reciprocity of an optical circuit. It is therefore possible to obtain an arrayed waveguide grating type optical multiplexer/demultiplexer in which the 1 dB band width is wide, the ripple is small, and the adjacent crosstalk is low also in the case in which a multi-mode waveguide having in at least a part thereof a waveguide whose width increases toward the arrayed waveguide is connected to the input end of each of the optical output waveguides. An example of such a waveguide with increasing width is a trapezoidal waveguide which is wider than the optical output waveguides and the width of which increases toward the arrayed waveguide.
The present invention described above does not have a complicated structure, but is simple. Therefore, manufacturing thereof is easy and it makes an arrayed waveguide grating type optical multiplexer/demultiplexer with high production yield.